The ion mobility spectrometer is a rapid, sensitive and portable apparatus based on ion mobility spectrometry for on-site detection. The apparatus can detect the presence of trace amounts of contrabands, such as explosives, drugs and chemical warfare agents, etc., and has currently found wide applications in the field of security inspection and military affairs.
The core component of the ion mobility spectrometer is an ion drift tube, where ambient air is generally used as the carrier gas. By adopting different sample introduction modes, the collected molecules of the substance to be detected in a gaseous state can be carried into the drift tube by air. In the ionization region of the drift tube, the air molecules are firstly ionized and form reactant ion clusters, then the molecules of substance to be detected interact with the reactant ion clusters to form new molecular ion clusters, i.e., product ion clusters. When the ion gate opens, these molecular ion clusters are injected into the drift region and continue to move forward under the influence of an electric field. In the drift region, the drift velocities of the molecular ion clusters depend on some factors including the mass, charges and spatial structure of the ion clusters so that their arrival times at the detector located at the end of the drift region are different from each other. The substance can be identified by measuring the weak current generated by the collision of the ion clusters onto the detector, determining their corresponding arrival times and matching the values with those recorded in the standard substance library.
The ionization of the gas molecules of the sample to be detected performed in the ionization region of the ion drift tube is a secondary ionization process: the concentration ratio of the carrier gas and the sample vapor causes the carrier gas molecules to be ionized by the ion source more easily than the sample vapor molecules, hence, the ionization process is first performed on the air molecules so as to generate reactant ions. Because the free path for ionized carrier gas molecules is much less than the geometric size of the reaction chamber, the ionized carrier gas molecules and the sample vapor molecules will collide frequently, so that the ion charges are transferred from the ionized carrier gas molecules to the sample molecules. Such transfer reaction between charges is decided by proton or electron affinity of the molecules participated in the reaction, the charges will transfer from molecules with small proton or electron affinity to molecules with large affinity. In actual use of the ion mobility spectrometer, the molecular composition and ionization mechanism of the reactant are generally changed by adding dopants, so as to change the chemical composition of the generated product ions and improve the detection sensitivity and selectivity of the apparatus. The molecules of the dopant must possess electron affinity lower than that of the sample molecules (such as the explosives) while higher than that of other components contained in the carrier gas, hence, it can be ionized first to generate stable ions so as to prevent the disruptors with lower affinity in the carrier gas from participating in the ionization reaction, meanwhile, because the electron affinity of the sample molecules is larger than the reagent molecules, these ions continue to react with the sample vapor molecules to generate sample molecular ions for detection. The adding of the dopant can also enable the spectrogram peak position to which the generated product ions correspond to shift and enable the ion peaks, that are difficult to be identified due to overlapping of peak positions when the dopant is not added previously, to be separated, so as to realize identification of the components to be detected in the case of existence of disruptors.
Currently, there have been many relevant patents that respectively describe the category selection and adding manner of the dopant in the ion mobility spectrometer or other similar analysis apparatus. The early patent such as EP0135747 describes that during detection of samples of drugs and explosives, the acetone and the carbon tetrachloride are added into the carrier gas as dopants before injection of the sample, the generated dimer ion or hydration Cl-ion can prevent inconstant composition cluster phenomenon of water, so that it forms a narrow peak with relatively fixed position on the ion spectrogram, and can be used as the reference point for electric control unit algorithm, so as to enable the apparatus to have better detection and identification specificity.
Relevant patent documents, such as WO2006123107, EP0509171, U.S. Pat. No. 5,283,199, U.S. Pat. No. 5,234,838, U.S. Pat. No. 5,032,721, DE19609582, DE10212110, WO2007085898, have described that the dopants of different categories that can be used in ion mobility spectrometry analysis are applicable for various detection requirements. For example, the use of the dopant containing di-pentanone improves the identification ability to some substances in the gas sample such as nitrogen compounds in the exhaled gas of mammals; adding a small amount of dopant of sulfur dioxide into the sample to be detected through a temperature control and permeable tube can eliminate the interference effect of acid gas components with relatively weak electron affinity such as hydrogen fluoride, sulfur dioxide, mefenamic acid, and improve the selectivity of the apparatus; similarly, using a small amount of substituted phenols (such as methyl salicylate, 2-hydrogen group acetophenone) and amines (such as methylamine) as the dopant can eliminate the interference effect of the chlorine gas; the added dopant dimethyl methylphosphonate (DMMP) can be used with the ammonia gas of the component to be detected to generate cluster ions the drift time of which is changed distinctly, so as to be easily identified from the spectrogram of the mixture gas; the aromatic compounds (benzene, toluene, xylene) added in the sample as the dopants have energy equal or less than that of the VUV ray photons and an ionization potential higher than that of the sample molecules, within the VUV radiation ionization range, it can be used to detect trace substances in the air, it may even detect substances with weak proton affinity only, and can improve the detection sensitivity of electronegative substances; ammonia water can be introduced into the analysis gas so as to separate the reactant ion peak from the formaldehyde peak, and then perform quantitative analysis of the formaldehyde; amides ionization dopants are used to detect peroxide explosives and the like.
On the adding manner of dopants, the patents concerned are PCT patent WO2006129101 and WO2004102611, the apparatus as stated in the former uses at least two reservoirs to provide various dopants, the reservoirs are connected with the ionization chamber of the spectrometer, an inlet is arranged at the side of the selectively permeable membrane facing the sample inlet, so that the sample gas contacts the dopant before the ionization, and the cycle gas circuit in the drift tube is separated from the doping gas circuit; the system described by the latter comprises a molecular sieve with dopants added therein, to which a first kind of dopant can be added successively. The system further comprises additional reservoirs filled with different dopants, other different dopants than the first kind of dopant are added into the air selectively via a switching arrangement. In addition, the apparatus involved in the U.S. Pat. No. 6,495,824 comprises a plurality of reservoirs filled with various dopants. The various dopants are selectively added into the carrier gas stream according to the variation of the detected signal, and react with the sample to generate additive products with different drift rates. An information library can be set up which contains known reaction information of the object to be detected and the various dopants, it can be determined whether the substance to be detected is contained indeed in the sample by comparing the observation result of the sample relative to a specific combination of a variety of dopants with the data in the information library. The patent WO2007082941 injects the object to be detected through the normal ionization interface at the inlet of the apparatus, and adds the additives into the foggy gas. US2002088936 combines the doping gas source with the drying and cleaning apparatus. U.S. Pat. No. 5,491,337 mixes the dopant with low concentration with the carrier gas in a closed container arranged before the sample and air inlet of the apparatus, which is introduced into the ionization chamber together with the sample gas. EP 1672363 mixes the sample gas with the doping gas before the sample gas enters into the apparatus, or adds the doping gas into the drift gas so as to eliminate the interference problem in analysis of the testing sample of a large amount of inert gas by using the ion mobility spectrometer.
In these prior art, the dopants for explosive detection are usually halohydrocarbon class, such as halogenated hydrocarbon. The container for filling dopants needs to have a flow rate control device such as temperature control and semi-permeable membrane, so as to generate sample gas with a specific dopant content and a constant flow rate. The apparatus may comprise two or more containers for filling dopants of different categories, selective adding of dopants of different categories is controlled by arrangements such as electromagnetic valves, so as to improve the ability of the apparatus to correctly identify the object to be detected under the existence of disruptors, or for detection requirements of different objects to be detected.
It is found in actual operations such as security inspection that the dopants has great improving function to the detection sensitivity and selectivity for some importance explosives such as RDX, PSTN, however, the adding of dopants cannot improve the detection capability for all explosives, for some explosives, such as DNT, black powder, relatively high detection sensitivity can be realized in the condition that no dopants exist, the adding of dopants such as chlorinated hydrocarbon molecules on the contrary will greatly reduce the sensitivity. Hence, if the conventional method of providing dopant dosage with a constant concentration by a temperature control and permeable tube is used, it is difficult to realize the optimal detection capability index for said various explosives simultaneously, besides, the preparation and replacement of the temperature control and permeable device increase the complexity and cost of the apparatus.